Oxytocin (OT) is a cyclic nona-peptide that mediates its physiological actions through activation of the oxytocin receptor (OT-R), a cell membrane receptor belonging to the class of G protein-coupled receptors that is similar to arginine vasopressin receptors. One important action of Oxytocin (OT) is to cause the contraction of the uterus of mammals during labor. Repeated, concerted and regular contraction of the uterus will cause the dilatation of the cervix, the rupture of fetal membranes and lead to expulsion of the fetus. Premature labor is when these contractions occur before the normal term of pregnancy. Preterm increase of uterine activity is the most common expression of preterm labor.
Premature labor leads to undesired premature birth, a serious health problem that remains the major cause of perinatal mortality and severe morbidity, especially respiratory distress syndrome, intraventricular haemorrhage, bronchopulmonary dysplasia and necrotising enterocolitis that are far more common in preterm than in term infants. Long-term impairments such as cerebral palsy, visual impairment and hearing loss are also more common in preterm infants. Nowadays, preterm birth remains the leading cause of infant mortality and morbidity in industrialized nations, where, despite the significant improvements in obstetrical medicine, it is causing high costs for neonatal intensive care of premature babies. The actual costs are even higher to society when taking into consideration the healthcare provision of preterm childbirth-related ailments, such as respiratory distress syndrome, heart conditions, cerebral palsy, epilepsy, and severe learning disabilities. The management of preterm labor represents a significant problem in the field of obstetrics.
The OT/OT-R system plays a vital role in initiating labor in mammals, in particular in humans. The density of OT-R increases markedly in the myometrium before the onset and during labor. Also it is thought that the local OT peptide hormone concentration increases markedly before parturition in human. The high circulating concentrations of progesterone induce uterine quiescence while the uterus acquires contractile ability. Shortly before term, plasma progesterone concentrations fall, OT-R expression in the uterus increases markedly, OT is released and uterine contractile activity increases. At term, the contractions rise to a crescendo, resulting in delivery as a result of two interacting positive feedback loops. The first is a local uterine loop: within the uterus itself, contractile prostaglandins are produced and released in response to OT and uterine contractions. These prostaglandins may play a further role in cervical ripening and weakening of fetal membranes. The second loop involves the hypothalamus: in response to uterine contractions and vaginal and cervical distension, magnocellular oxytocin neurons in the hypothalamus increase their activity resulting in the release of OT from their axon terminals in the posterior pituitary. The released OT acts upon the uterus both to stimulate the further production of prostaglandins and to contribute further to the contractions of the uterus.
Therefore, blocking the effect of OT by antagonizing OT-R might represent an attractive modality for the treatment of diseases related to the OT-R activity, in particular preterm labor.
Tocolytic, i.e. uterus relaxing agents, have been used in clinical studies for the pharmaceutical treatment of preterm labor. Most of these agents are used off-label. They have shown very limited efficacy, if any, in prolonging gestation and without clear demonstration of improvement of neonate outcome. Current tocolytics are very often associated with unwanted adverse effects on women, foetus or neonate. Such tocolytics include beta-2-adrenergic agonists, prostaglandin synthesis inhibitors, magnesium sulfate, nitric acid donors and calcium channel blockers. Beta-2-adrenergic agonists such as ritodrine or terbutaline cause a number of cardiovascular and metabolic side effects including maternal tachycardia, palpitations, hypotension, altered thyroid function and fetal and neonatal hypoglycaemia, tachycardia. Ritodrine is no longer FDA approved. The calcium channel blocker nifedipine is also a medicine that is used to try to stop contractions. Some of the side effects that may occur include facial flushing, headache, nausea, palpitations, and lightheadedness. The total prostaglandin synthesis inhibitor (NSAID) indomethacin has been used. It can also have serious effects on the fetus: constriction of ductus arteriosus, pulmonary hypertension, decrease in renal function with oligohydramnios, intraventricular hemorrhage, hyperbilirubinemia, necrotizing enterocolitis. Maternal side effects include abdominal discomfort, nausea, vomiting, depression and dizzy spells for the mother. Another NSAID is sulindac that has a side effect profile similar to indomethacin. For magnesium sulfate, meta-analyses have failed to support it as a tocolytic agent. Women reported important side effects such as flushing, lethargy, headache, muscle weakness, pulmonary edema and cardiac arrest. A newborn who has been exposed to magnesium sulfate may show lethargy, hypotonia, respiratory depression, bone problems, osteopenia and fractures. Recently, the FDA is advising healthcare professionals against using magnesium sulfate injection for longer than 5-7 days to stop preterm labor in women.
Atosiban, a dual vasopressin V1a receptor and OT-R antagonist is marketed in EU and used to stop contractions and delay preterm delivery by a few days. Atosiban is a peptide that is not orally bioavailable and must be administered parenterally. It is rapidly degraded in circulation by enzymes and its use is limited to maximum 48 h.
In addition, non-peptide OT-R antagonists were developed such as pyrrolidine derivatives (WO 01/72705, WO 02/102799, WO 2002/074741, WO 2004/005249) as mixtures of isomers.
There remain significant unmet needs for efficient and orally selective OT-R antagonist for the treatment of diseases related to the OT-R activity, in particular preterm labor.